This invention relates to improved dynamic shaft seals for submersible motors and more particularly to improved dynamic sealing means for sealing the motor shaft to the motor housing of a submersible pump so that there is no leakage of external or well fluids into the pump motor.
Centrifugal fluid seals are widely used between a relatively rotatable shaft and a housing having a fluid chamber surrounding an opening in which the shaft is sealed. These liquid barrier seals have the ability to provide a leakproof seal under relatively high pessure differentials and high shaft speeds with relatively low friction losses. High sealing pressures are acheived with centrifugal seals only at high rotating speeds, however, and sealing at startup and at low rotational speeds is difficult. Additionally, where employed to seal shafts which operate in a vertical position, preventing escape of seal fluid from the fluid chamber presents a major problem.
A variety of seal configurations and combination seals have been devised to provide sealing at low shaft speeds and to prevent loss of seal fluid from the seal chamber when the shaft is not rotating, particularly for use in vertical shaft appplications. In U.S. Pat. No. 2,295,579 there is shown a mercury-filled centrifugal seal for a vertical shaft employing an elaborate labyrinth to retain mercury in the seal and to maintain effective contact of the mercury with the shaft at all rotational velocities. Seals of this type suffer several disadvantages, including the requirement for complex and expensive machining of parts and the use of potentially toxic mercury.
An alternative design also employed with vertical shaft installations includes a mechanical static seal disposed axially along the shaft below the centrifugal seal, such as is shown for example in U.S. Pat. No. 2,781,209. The mechanical seal operates at zero or slow rotational speeds to retain the centrifugal seal fluid in the seal fluid chamber and to prevent leakage of seal fluid and external or pumped fluids through the shaft seal area. At high rotational speeds, where the centrifugal seal becomes effective, the mechanical seal is disengaged from contact with the shaft by centrifugal forces. As with other mechanical seals, the primary disadvantages of such combination seals are the high failure rate caused by wear when operating under extreme conditions of speed, pressure and corrosive environments, increased frictional losses due to the mechanical seal, the requirement for complex mechanical design and the need for additional machining operations.
A completely dynamic seal for use with vertical shaft installations which would be capable of sealing against large pressure differentials and which would operate to minimize or prevent loss of seal fluids at low rotational speeds and which would avoid the need for use of potentially toxic seal fluids such as mercury would be a desirable improvement over presently available dynamic seals.